Tertiary amines are industrially significant compounds. They can serve as polymerization and curing catalysts for the production of epoxy and urethane-based plastics. Moreover, they are suitable as corrosion inhibitors and adsorption agents for synthesis gas scrubbing. This applies in particular to the easily prepared dimethyl derivatives.
The hydrogenating methylation of primary amines with formaldehyde and hydrogen is an important synthesis route to the preparation of methylated amines. A summary of this process is to be found in Houben-Weyl, Methoden der organischen Chemie; Vol. XI/1, pages 641 to 643, 4th edition (1957).
According to DE-OS 19 32 422 the reaction can be performed using conventional fixed-bed catalysts in liquid phase. Prior application DE 37 21 539 describes a modified process. Here, the starting materials (amine, formaldehyde and hydrogen) are heated separately to a specified temperature and then mixed in the presence of a fixed-bed catalyst. The starting materials, in particular the formaldehyde, are to contain a reduced amount of water. The fixed catalysts are placed in tubular reactors which can withstand the necessary high pressures.
However, it is also possible to dispense with the use of complicated high-pressure tubular reactors and to perform the reaction, for example, in vessels with stirring or loop reactors. In this case, the hydrogenating N-methylation is conducted with the aid of a suspended hydrogenation catalyst. This process is also called the suspension method; the present invention relates to this process variation.
After prolonged use, the hydrogenation catalysts decompose to an increasing extent. The resultant fine-grained particles are undesirable as they make the separation of the suspended catalyst after reaction more difficult. Normally, the suspended catalyst is removed by sedimentation, centrifugation, and/or filtration. The finer the catalyst particles are, the more difficult it is to remove them from the reaction mixture. A smaller particle size means a reduction in the rate of sedimentation, and also has a negative effect on centrifugation. If filtration is employed to separate the catalyst, the particularly small particles quickly block the filtration unit by clogging the filter pores. This causes an increase in pressure in the apparatus with the result that filtration has to be interrupted to clean the filter.
The presence of formic acid, which probably forms from formaldehyde through the Cannizzaro reaction, is also undesirable. It removes a corresponding amount of amine from the reaction in the form of a salt. In addition, the free acid and the amine salt promote corrosion in the reactor system.
Further disruptive side reactions are a result of the polymerization of formaldehyde with itself and the polycondensation between the amine and the formaldehyde to hexahydrotriazines or, in the case of multivalent amines, higher molecular compounds. The formation of polymeric substances produces a reduction in the catalyst activity owing to caking of the suspended catalyst and can even lead to the reactor being blocked. The unavoidable consequence is that the catalyst has to be changed.
The EP-A 0 142 868 recommends, for the N-alkylation of amines, the use of special hydrogenation catalysts which contain at least one of the elements Co, Ni, Ru, Rh, Pd and Pt on carbon as a carrier. The catalyst is suspended in the amine and the carbonyl compound added continuously. Aluminum oxide, silicon dioxide, and siliceous earth are undesirable as carriers for the hydrogenation catalysts since they do not ensure good distribution of the catalyst in the reaction system. Although metal catalysts without carriers, such as Raney nickel, Raney cobalt, palladium black, and platinum black are very active, they do not produce good results, as comparative tests have shown.
The teachings of ES 538 216 are comparable. While Pd and Pt-containing catalysts give good results on activated carbon, Raney nickel and a standard nickel carrier catalyst produce highly unfavorable results in the N-methylation of primary amines. This has been confirmed in comparative tests.
As can be seen from the preceding statements, the successful use of nickel-containing catalysts for the reaction of primary amines with formaldehyde and hydrogen is limited to special nickel-activated carbon carrier catalysts.